1. Technical Field
The present invention generally relates to alkali metal thermal to electric conversion (AMTEC) cells and more particularly to a heat shield for such a cell for reducing the radiative heat transfer between the heat input zone of the cell and the heat rejection zone of the cell.
2. Discussion
An AMTEC cell is a thermally regenerative concentration cell typically utilizing an alkali metal, such as sodium or potassium, as a working fluid and a solid electrolyte as an ion selective membrane. While throughout this description sodium is referred to as the working fluid, it is to be understood that other alkali metals are also applicable to this invention. The electrolyte separates the cell into a high pressure zone and a low pressure zone while permitting a nearly isothermal expansion of the alkali metal for generating high-current/low voltage power at high efficiency. To accomplish this, most AMTEC cells employ at least one beta-alumina type solid electrolyte (BASE) element, commonly shaped as a tube, which is exposed to high-pressure sodium on one surface and low-pressure sodium on an opposite surface. A high-temperature evaporator near the "hot" end of the cell produces the high pressure and a low-temperature condenser at the "cold" end of the cell maintains the low-pressure.
The BASE element's opposed surfaces are overlaid with permeable electrodes which are connected to each other through an external load circuit. Neutral sodium atoms incident on one of the BASE element's surfaces release their electrons to one electrode (the anode). The resulting sodium ions pass through the element wall under the applied pressure gradient, and the emerging sodium ions are neutralized at the other electrode (the cathode) by electrons returning from the external load. As such, the pressure gradient drives the sodium through the BASE element thereby creating an electrical current which passes through the external load resistance.
The neutral sodium vapor at the outer electrodes migrates through the chamber between the BASE element and the cell wall until it condenses at the low-temperature condenser at the cold end of the cell. From there, the sodium condensate flows through an artery containing a fine pore membrane commonly consisting of a packed metallic felt toward the high pressure zone at the hot end of the cell. The liquid sodium evaporates at the high temperature evaporator which is coupled to the artery and is returned to the inside of the BASE element through a common plenum at the hot end of the cell.
The heat transfer that occurs between the heat input zone of the cell proximate the hot end and the heat rejection zone of the cell proximate the cold end, other than the latent heat of the working fluid condensing at the cold end of the cell, is referred to as parasitic heat transfer. In a typical AMTEC cell operating at two amperes of electrical current, the percentage of the energy that is parasitic in nature is approximately 48%. Of this parasitic energy, approximately 12.5% is radiative heat transfer directly to the condenser surface and approximately 87.5% is conductive heat transfer from the cell wall to the condenser surface.
It has now been found that cell performance can be enhanced by lowering the radiative heat transfer between the heat input zone of the cell and the heat rejection zone of the cell. Cell performance can also be enhanced by increasing the operating temperature of the BASE element. Therefore, it would be desirable to provide a heat shield for reducing the amount of radiative heat transfer between the hot and cold ends of the cell while increasing the cell efficiency by raising the temperature of the BASE element.